Device and method for polishing a specimen

ABSTRACT

The present invention relates to a device for polishing a specimen ( 104, 204 ). The device comprises a polishing platen ( 101, 201 ) rotatable about an axis, a polishing pad ( 103, 203 ) attached to the polishing platen ( 101, 201 ), a specimen holder ( 105, 205 ) for holding the specimen ( 104, 204 ) against the polishing pad ( 103, 203 ), means ( 107, 110, 207, 212 ) for measuring a physical quantity in a plurality of positions on the polishing pad ( 103, 203 ), the physical quantity being indicative of the moisture or the friction, and means ( 116, 119, 120, 121, 213, 215, 216, 217 ) for dispensing a polishing suspension, based on values of the measured physical quantity, to the plurality of positions on the polishing pad ( 103, 203 ). The invention also relates to a method for polishing a specimen ( 104, 204 ).

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a device and a method for polishing aspecimen according to the preambles of the appended independent claims.

BACKGROUND OF THE INVENTION

Various polishing devices are known in the prior art for polishing rock,metallographic and other specimens of solid materials for differentapplications. The polishing devices enable to polish specimens withminimal manual work.

An exemplary polishing device comprises a rotatable polishing platen anda polishing pad that is attached to the surface of the polishing platen.The polishing device also comprises a specimen holder for holding one ormore specimens against the polishing pad. As the polishing platen isrotated, the polishing pad polishes the specimen(s).

A polishing suspension is typically used in the polishing device toenhance the polishing action and to prevent overheating during thepolishing process. The polishing suspension contains water and/or othersuitable liquids, such as ethanol or glycol for moistening, cooling andlubricating the polishing pad and abrasive particles, such as diamond,aluminium oxide or colloidal silica for polishing of the surface of thespecimen. The polishing device may comprise a dispenser for dispensingthe polishing suspension at a predefined rate on the polishing pad. Thedispenser can, for example, be configured to dispense a certain amountof the polishing suspension at regular time intervals on the polishingpad.

A problem associated with the known polishing devices is that, duringthe polishing, the moisture in the polishing pad can vary considerablyas a function of time and as a function of the position on the polishingpad. This has a negative effect on the polishing result. If thepolishing pad is too wet, the friction between the specimen and thepolishing pad is small, which makes the polishing ineffective andincreases the polishing time. If, on the other hand, the polishing padis too dry, there is a risk that the polishing of the specimen fails dueto the large friction between the specimen and the polishing pad.

OBJECTIVES OF THE INVENTION

It is the main objective of the present invention to reduce or eveneliminate the prior art problems presented above.

It is an objective of the present invention to provide a device and amethod for polishing a specimen. In more detail, it is an objective ofthe invention to provide a device and a method enabling to provide andmaintain a substantially constant and uniform moisture content in thepolishing pad during the polishing process. It is a further objective ofthe invention to provide a device and a method enabling to polish aspecimen with a uniform and high-quality polishing result.

In order to realise the above-mentioned objectives, the device and themethod according to the invention are characterised by what is presentedin the characterising portions of the appended independent claims.Advantageous embodiments of the invention are described in the dependentclaims.

DESCRIPTION OF THE INVENTION

A device according to the invention for polishing a specimen comprises apolishing platen rotatable about an axis, a polishing pad attached tothe polishing platen, a specimen holder for holding the specimen againstthe polishing pad, means for measuring a physical quantity in aplurality of positions on the polishing pad, the physical quantity beingindicative of the moisture or the friction, and means for dispensing apolishing suspension, based on values of the measured physical quantity,to the plurality of positions on the polishing pad.

The device according to the invention, i.e. the polishing device, can beused to polish a specimen (i.e. an object) of a solid material, such asa piece of a natural (e.g. rock, mineral, ore or meteorite) orartificial (e.g. metal, concrete, ceramic, composite or semiconductor)material. In this text, the term polishing means polishing,fine-grinding and grinding processes with a polishing pad and apolishing suspension.

The specimen to be polished is held against the surface of the polishingpad with the specimen holder. The specimen holder can be configured tohold one or more specimens against the polishing pad. The number ofspecimens that can be held with the specimen holder against thepolishing pad can be, for example, 1, 2-5, 6-10, or more than 10. Thespecimen holder can comprise, for example, one or more inserts in whichthe specimens can move freely or are secured by clamping. The specimenholder can be attached to a polishing head that presses the specimen(s)with an adjustable pressing force against the polishing pad. Thepressing force can be adjusted, for example, based on the values of themeasured physical quantity.

The polishing pad can be made of different materials, such as a fibrouscloth (e.g. real silk, polyester or acetate) or synthetic non-fibrous(e.g. neoprene, polyurethane or composite) material. The polishing padis preferably disc-shaped, i.e. it has a flat circular shape. Thepolishing pad is preferably porous, enabling it to absorb the polishingsuspension. The diameter of the polishing pad can be, for example, lessthan 500 mm, preferably 200-400 mm. The thickness of the polishing padcan be, for example, less than 3 mm, preferably less than 2 mm, and morepreferably less than 1 mm.

The polishing pad is attached, preferably releasably, to the surface ofthe rotatable polishing platen. As the polishing platen is rotated, thepolishing pad polishes the specimen. The polishing platen can be made ofa heat conductive and corrosion resistant material, such as aluminium,bronze or stainless steel, or of a plastic, polymer or compositematerial. The polishing platen may comprise an electrically insulatedelectrical contact element (for example a sheet made of austeniticstainless steel and having a thickness of about 0.5 mm) on its surface,to which an electrical contact element (for example a carbon brush) of ameasuring instrument can be electrically connected. The polishing platenis preferably disc-shaped and heat conductive. The diameter of thepolishing platen can be, for example, less than 500 mm, preferably200-400 mm. Preferably, the diameters of the polishing platen and thepolishing pad are essentially the same. The thickness of the polishingplaten can be, for example, 20-50 mm. The optimal thickness of thepolishing platen depends on the diameter and the material of thepolishing platen.

The polishing device may comprise an actuator, such as an electricmotor, for rotating the polishing platen. The actuator can be configuredto rotate the polishing platen, for example, at a rotational speed of0-600 rpm. The actuator can be configured to adjust the rotational speedof the polishing platen based on the values of the measured physicalquantity. For example, in a case where the friction exceeds apredetermined threshold value or the moisture falls below apredetermined threshold value, the actuator can be configured to reducethe rotational speed of the polishing platen. To reduce possibleinterferences caused by the actuator, it may be beneficial to add anelectrical insulation layer between the polishing platen and theactuator.

In addition to the polishing platen, also the specimen holder can berotatable. The specimen holder can be rotatable about an axis, which isparallel to the rotation axis of the polishing platen. The polishingdevice may comprise an actuator, such as an electric motor, for rotatingthe specimen holder. The actuator can be configured to rotate thespecimen holder, for example, at a rotational speed of 0-150 rpm. Theactuator can be configured to adjust the rotational speed of thespecimen holder based on the values of the measured physical quantity.For example, in a case where the friction exceeds a predeterminedthreshold value or the moisture falls below a predetermined thresholdvalue, the actuator can be configured to reduce the rotational speed ofthe specimen holder.

The device according to the invention can measure the physical quantity,which is indicative of the moisture or the friction, in a plurality ofpositions on the polishing pad and, based on the values of the measuredphysical quantity, dispense the polishing suspension to the plurality ofpositions on the polishing pad.

The means for measuring the physical quantity may comprise one or moremovable sensors, or a plurality of fixed (non-movable) sensors arrangedat a predetermined distance from each other. The sensor can be amoisture sensor, which can measure, for example, the (electrical)resistance on the polishing pad or the capacitance between the oppositesides of the polishing pad (i.e. the capacitance over the thickness ofthe polishing pad), which both vary as a function of the moisture in thepolishing pad. The sensor can also be a force sensor, which isconfigured to measure the friction on the surface of the polishing pad.The measurement of the friction can be based on detecting the lateralforce acting on an object that is arranged in contact with the surfaceof the polishing pad.

The means for dispensing the polishing suspension may comprise one ormore movable spray nozzles, or a plurality of fixed (non-movable) spraynozzles arranged at a predetermined distance from each other. The meansfor dispensing the polishing suspension may also comprise a containerfor the polishing suspension. Alternatively, the means for dispensingthe polishing suspension may comprise a plurality of containers for thepolishing suspension or for different polishing suspensions. Thecontainer(s) can be connected by flexible hoses to the spray nozzles.

The means for dispensing the polishing suspension may comprise a controlunit for controlling the spray nozzles. The control unit may comprise aprocessor and a memory including computer program code, the memory andthe computer program code being configured to, with the processor, causethe spray nozzles to dispense suitable amounts of the polishingsuspension to the plurality of positions on the polishing pad. Theamount of the polishing suspension to be dispensed to each position isdetermined based on the values of the measured physical quantity. Thevalues of the measured physical quantity can be compared to a predefinedreference value or profile in order to determine the amount of thepolishing suspension to be dispensed. The predefined reference value orprofile corresponds to the desired moisture content in the polishingpad. The amount of the polishing suspension to be dispensed to eachposition on the polishing pad can be, for example, less than 5 μl/cm²,less than 2 μl/cm² or 0.5-2 μl/cm².

The polishing suspension enhances the polishing action and preventsoverheating during the polishing process. The polishing suspensioncontains water and/or other suitable liquids, such as alcohols (e.g.ethanol, glycol or isopropyl alcohol) for moistening, cooling andlubricating the polishing pad. The polishing suspension may also containabrasive particles, such as diamonds, aluminium oxide or colloidalsilica for polishing of the surface of the specimen. Typically, thepolishing suspension (i.e. slurry or fluid) is a mixture of liquid(s)and abrasive(s).

An advantage of the device according to the invention is that it canprovide and maintain a substantially constant and uniform moisturecontent in the polishing pad during the polishing process. Anotheradvantage of the device according to the invention is that it enables toachieve a uniform and high-quality polishing result of the specimen.Still another advantage of the device according to the invention is thatit enables to optimize (minimize) the consumption of the polishingsuspension in the polishing process.

According to an embodiment of the invention the physical quantity is theresistance on the polishing pad and the means for dispensing a polishingsuspension is configured to dispense the polishing suspension to aposition on the polishing pad if in said position the value of theresistance is larger than a first threshold value. The resistance on thepolishing pad varies as a function of the moisture in the polishing pad.The wetter the polishing pad is, the smaller the resistance is.

According to an embodiment of the invention the physical quantity is thecapacitance between the opposite sides of the polishing pad and themeans for dispensing a polishing suspension is configured to dispensethe polishing suspension to a position on the polishing pad if in saidposition the value of the capacitance is smaller than a second thresholdvalue. The capacitance between the opposite sides of the polishing padvaries as a function of the moisture in the polishing pad. The wetterthe polishing pad is, the larger the capacitance is.

According to an embodiment of the invention the physical quantity is thefriction on the surface of the polishing pad and the means fordispensing a polishing suspension is configured to dispense thepolishing suspension to a position on the polishing pad if in saidposition the value of the friction is larger than a third thresholdvalue. The friction on the surface of the polishing pad varies as afunction of the moisture in the polishing pad. The wetter the polishingpad is, the smaller the friction is.

According to an embodiment of the invention the means for measuring aphysical quantity comprises a sensor arranged in contact with thesurface of the polishing pad and means for moving the sensor between theplurality of positions on the polishing pad. The means for moving thesensor is configured to move the sensor along the surface of thepolishing pad. The sensor can be a moisture sensor, which measures theresistance on the polishing pad and/or the capacitance verticallybetween the opposite sides of the polishing pad. The sensor canalternatively be a force sensor, which measures the friction on thesurface of the polishing pad.

According to an embodiment of the invention the means for moving thesensor comprises an arm arranged above the polishing pad and an actuatorfor moving the sensor along the arm. The arm is preferably straight, andits longitudinal axis is arranged parallel to the plane of the polishingpad. The arm is preferably arranged above the polishing pad in such amanner that it extends from the centre to the periphery of the polishingpad. This enables the sensor to be moved to all possible positions onthe polishing pad as the polishing platen is rotated. The actuator canbe an electric motor.

According to an embodiment of the invention the means for moving thesensor is the specimen holder that is configured to hold the sensoragainst the polishing pad. The specimen holder can comprise, forexample, an insert in which the sensor can move freely or is secured byclamping. The sensor may comprise a wireless transmitter or transceiverfor communicating with a remote device, such as a measuring instrumentand/or a control unit that controls the spray nozzle(s).

According to an embodiment of the invention the means for measuring aphysical quantity comprises an array of sensors arranged in contact withthe surface of the polishing pad. The sensors can be attached to an armthat is arranged above the polishing pad. Preferably, the sensors areattached to the arm at a predetermined distance from each other. The armis preferably straight, and its longitudinal axis is arranged parallelto the plane of the polishing pad. The arm is preferably arranged abovethe polishing pad in such a manner that it extends from the centre tothe periphery of the polishing pad. The number of sensors depends on thediameter of the polishing pad and the needed measurement accuracy, andit can be, for example, less than 100, less than 50, less than 20, or6-12. The sensors can be moisture sensors, which measure the resistanceon the polishing pad and/or the capacitance between the opposite sidesof the polishing pad. The sensors can alternatively be force sensors,which measure the friction on the surface of the polishing pad.

According to an embodiment of the invention the sensor comprises atleast two electrical contact elements arranged in contact with thesurface of the polishing pad, and the means for measuring a physicalquantity comprises means for measuring the resistance between two of theat least two electrical contact elements. The electrical contactelements, i.e. the electrodes, provide an electrical path through whichelectrical properties of the polishing pad can be measured. Theelectrical contact elements can have different shapes and sizes. Theelectrical contact elements are made of an electrically conductivematerial, which is preferably also wear and corrosion resistant.Preferable materials for the electrical contact elements are, forexample, stainless steel and tool steel. The electrical contact elementsare electrically isolated from each other to prevent short circuits. Themeans for measuring the resistance between two of the at least twoelectrical contact elements may comprise a measuring instrument that iselectrically connected with electrical connections, such as wires,cables or carbon brushes, to two electrical contact elements of thesensor. The measuring instrument is configured to measure the resistancebetween these two electrical contact elements.

According to an embodiment of the invention the means for measuring aphysical quantity comprises means for measuring the capacitance betweenone of the at least two electrical contact elements and the polishingplaten. The means for measuring the capacitance between one of the atleast two electrical contact elements and the polishing platen maycomprise a measuring instrument that is electrically connected withelectrical connections, such as wires, cables or carbon brushes, to theelectrical contact element and the polishing platen. The measuringinstrument is configured to measure the capacitance between theelectrical contact element and the polishing platen. A resistor can beused between the measuring instrument and the polishing platen to reduceinterference.

According to an embodiment of the invention the means for measuring aphysical quantity comprises a compression element for pressing thesensor against the surface of the polishing pad. The compression elementenables to keep the sensor in contact with the surface of the polishingpad with a suitable pressing force. The pressing force of thecompression element can be adjustable. The compression element can be,for example, a spring. The compression element can utilize air pressure,gravity, or magnetic or electromagnetic force in producing the suitablepressing force.

According to an embodiment of the invention the means for measuring aphysical quantity comprises means for measuring the lateral force actingon the sensor. The means for measuring the lateral force may comprise aforce sensor that is attached to the sensor.

According to an embodiment of the invention the means for measuring aphysical quantity comprises a contactless moisture sensor arranged abovethe polishing pad and means for moving the contactless moisture sensorbetween the plurality of positions on the polishing pad. The contactlessmoisture sensor can measure the moisture in the polishing pad. Thecontactless moisture sensor can be, for example, an optical sensor. Themeans for moving the contactless moisture sensor may comprise an armarranged above the polishing pad and an actuator for moving thecontactless moisture sensor along the arm.

According to an embodiment of the invention the means for measuring aphysical quantity comprises an array of contactless moisture sensorsarranged above the polishing pad. The contactless moisture sensors canmeasure the moisture in the polishing pad. The contactless moisturesensors can be, for example, optical sensors. The contactless moisturesensors can be attached to an arm that is arranged above the polishingpad. Preferably, the contactless moisture sensors are attached to thearm at a predetermined distance from each other.

According to an embodiment of the invention the contactless moisturesensor is a spectrometer that is configured to measure the intensity oflight reflected from the surface of the polishing pad as a function ofthe wavelength. The intensity of the reflected light depends on themoisture in the polishing pad. Depending on the used polishingsuspension and the material of the polishing pad, the intensity of thereflected light at a certain wavelength can act quite differently as themoisture in the polishing pad changes. For example, when the moisture inthe polishing pad decreases, the intensity of the reflected light canincrease at one wavelength and decrease at the other wavelength. Thespectrometer may comprise a plurality of optical detectors, each of thembeing adapted to detect light at a different wavelength. The wavelengthrange of the spectrometer can be, for example, 400-700 nm, 300-1000 nmor 750-2500 nm. Preferably, the distance between the spectrometer andthe surface of the polishing pad is kept constant during themeasurement. The surface of the polishing pad can be illuminated withone or more light sources, such as a light-emitting diode (LED).

According to an embodiment of the invention the contactless moisturesensor is a polymer membrane. The resistance of the polymer membranedepends on the moisture. The moisture in the polishing pad can bemeasured by arranging the polymer membrane close to the surface of thepolishing pad. For the measurement of the resistance, the polymermembrane can be provided with two electrical contact elements. Thepolymer membrane is preferably attached to a substrate, such as a glasspiece.

According to an embodiment of the invention the device comprises meansfor measuring the temperature of the sensor and/or the polishing pad,and means for adjusting the rotational speed of the polishing platenand/or the specimen holder based on a value of the measured temperature.The means for measuring the temperature may comprise contact and/orcontactless temperature sensors.

According to an embodiment of the invention the device comprises meansfor measuring the vibration of the sensor and means for adjusting therotational speed of the polishing platen and/or the specimen holderbased on a value of the measured vibration. The means for measuring thevibration may comprise a vibration sensor that is attached to thesensor.

According to an embodiment of the invention the device comprises meansfor cooling the polishing platen. The polishing platen can be cooled byspraying water onto the bottom side of the polishing platen. The devicemay comprise a splash guard to prevent the mist of cooling waterescaping onto the polishing pad. The splash guard can be made of, forexample, plastic.

According to an embodiment of the invention the means for dispensing apolishing suspension comprises a spray nozzle arranged above thepolishing pad and means for moving the spray nozzle between theplurality of positions on the polishing pad. The means for dispensing apolishing suspension may comprise one or more containers for thepolishing suspension or for different polishing suspensions. Thecontainer can be connected, for example, by a flexible hose to the spraynozzle.

According to an embodiment of the invention the means for moving thespray nozzle comprises an arm arranged above the polishing pad and anactuator for moving the spray nozzle along the arm. The arm ispreferably straight, and its longitudinal axis is arranged parallel tothe plane of the polishing pad. The arm is preferably arranged above thepolishing pad in such a manner that it extends from the centre to theperiphery of the polishing pad. This enables the spray nozzle to bemoved to all possible positions on the polishing pad as the polishingplaten is rotated. The actuator can be an electric motor.

According to an embodiment of the invention the means for moving thespray nozzle is the specimen holder to which the spray nozzle isattached.

According to an embodiment of the invention the means for dispensing apolishing suspension comprises an array of spray nozzles arranged abovethe polishing pad. The spray nozzles can be attached to an arm.Preferably, the spray nozzles are attached to the arm at a predetermineddistance from each other. The arm is preferably straight, and itslongitudinal axis is arranged parallel to the plane of the polishingpad. The arm is preferably arranged above the polishing pad in such amanner that it extends from the centre to the periphery of the polishingpad. The number of spray nozzles depends on the diameter of thepolishing pad, and it can be, for example, less than 100, less than 50,less than 20, or 6-12. The means for dispensing a polishing suspensionmay comprise one or more containers for the polishing suspension or fordifferent polishing suspensions. The container(s) can be connected, forexample, by flexible hoses to the spray nozzles.

According to an embodiment of the invention the device comprises meansfor evaporating the polishing suspension from the polishing pad. Thepolishing suspension can be evaporated with a stream of compressed airto a desired position on the polishing pad.

The present invention also relates to a method for polishing a specimen.The method according to the invention comprises holding the specimenwith a specimen holder against a polishing pad that is attached to apolishing platen, rotating the polishing platen about an axis, measuringa physical quantity indicative of the moisture or the friction in aplurality of positions on the polishing pad, and dispensing a polishingsuspension, based on values of the measured physical quantity, to theplurality of positions on the polishing pad.

An advantage of the method according to the invention is that it canprovide and maintain a substantially constant and uniform moisturecontent in the polishing pad during the polishing process. Anotheradvantage of the method according to the invention is that it enables toachieve a uniform and high-quality polishing result of the specimen.Still another advantage of the method according to the invention is thatit enables to consume less polishing suspension during the polishingprocess.

The exemplary embodiments of the invention presented in this text arenot interpreted to pose limitations to the applicability of the appendedclaims. The verb “to comprise” is used in this text as an openlimitation that does not exclude the existence of also unrecitedfeatures. The features recited in the dependent claims are mutuallyfreely combinable unless otherwise explicitly stated.

The exemplary embodiments presented in this text and their advantagesrelate by applicable parts to the device as well as the method accordingto the invention, even though this is not always separately mentioned.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a polishing device according to a first embodiment ofthe invention, and

FIG. 2 illustrates a polishing device according to a second embodimentof the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a polishing device according to a first embodiment ofthe invention. The polishing device comprises a polishing platen 101that can be rotated about an axis with an actuator 102. The actuator 102can rotate the polishing platen 101 at different rotational speeds. Apolishing pad 103 is attached to the polishing platen 101. The polishingpad 103 is used for polishing a specimen 104 that is held against thepolishing pad 103 with a specimen holder 105. The specimen holder 105can be rotated about an axis, which is parallel to the rotation axis ofthe polishing platen 101, with an actuator 106. The actuator 106 canrotate the specimen holder 105 at different rotational speeds. As thepolishing platen 101 and the specimen holder 105 are rotated, thepolishing pad 103 polishes the specimen 104.

The polishing device comprises a moisture sensor 107 for detecting themoisture in the polishing pad 103. The moisture sensor 107 compriseselectrical contact elements 108 and 109, which are arranged in contactwith the surface of the polishing pad 103. The electrical contactelements 108 and 109 provide an electrical path through which electricalproperties of the polishing pad 103 can be measured. The electricalcontact elements 108 and 109 are electrically connected to a measuringinstrument 110 that can measure the resistance between the electricalcontact elements 108 and 109. The resistance between the electricalcontact elements 108 and 109 is indicative of the moisture in ameasurement position on the polishing pad 103. The measuring instrument110 is electrically connected to the polishing platen 101, thus allowingto measure the capacitance between one of the electrical contactelements 108, 109 and the polishing platen 101, i.e. between theopposite sides of the polishing pad 103. The capacitance between theelectrical contact element 108 or 109 and the polishing platen 101 isindicative of the moisture in a measurement position on the polishingpad 103.

The moisture sensor 107 is attached with a rod 111 to an arm 112 that islocated above the polishing pad 103. The arm 112 extends from the centreto the periphery of the polishing pad 103. The moisture sensor 107 canbe moved with an actuator 113 along the arm 112, thus enabling to movethe moisture sensor 107 between different positions on the polishing pad103. The rod 111 is provided with a spring 114 for pressing the moisturesensor 107 against the surface of the polishing pad 103. The spring 114enables to keep the electrical contact elements 108 and 109 in contactwith the surface of the polishing pad 103 with a suitable pressingforce.

The polishing device comprises a force sensor 115 for detecting thelateral force acting on the moisture sensor 107. The force sensor 115 isattached to the rod 111 and electrically connected to the measuringinstrument 110 that can determine the lateral force, i.e. the frictionon the surface of the polishing pad 103. The friction on the surface ofthe polishing pad 103 is indicative of the moisture in a measurementposition on the polishing pad 103.

The polishing device comprises a spray nozzle 116 for dispensing apolishing suspension on the polishing pad 103. The spray nozzle 116 isattached to an arm 117 that is located above the polishing pad 103. Thearm 117 extends from the centre to the periphery of the polishing pad103. The spray nozzle 116 can be moved with an actuator 118 along thearm 117, thus enabling to move the spray nozzle 116 between differentpositions above the polishing pad 103.

The polishing suspension is stored in a container 119 from which it isconveyed through a flexible hose 120 to the spray nozzle 116 to bedispensed on the polishing pad 103. The polishing device comprises acontrol unit 121 for controlling the dispensing of the polishingsuspension. The control unit 121 controls the spray nozzle 116 todispense suitable amounts of the polishing suspension to desiredpositions on the polishing pad 103. The amount of the polishingsuspension to be dispensed to each position is determined based on thedetermined moisture or the friction on each position.

The polishing device comprises a temperature sensor 122 attached to theelectrical contact element 108 for measuring the temperature of theelectrical contact element 108, and a temperature sensor 123 attached tothe arm 112 for contactlessly measuring the temperature of the polishingpad 103. The polishing device also comprises a vibration sensor 124attached to the rod 111 for measuring the vibration of the moisturesensor 107. The control unit 121 can control the rotational speeds ofthe polishing platen 101 and the specimen holder 105 based on themeasured temperatures and/or vibration.

The polishing device comprises a spray nozzle 125 for spraying wateronto the bottom side of the polishing platen 101 in order to cool thepolishing platen 101. A splash guard 126 is used to prevent the mist ofwater escaping onto the polishing pad 103.

FIG. 2 illustrates a polishing device according to a second embodimentof the invention. The polishing device comprises a polishing platen 201that can be rotated about an axis with an actuator 202. The actuator 202can rotate the polishing platen 201 at different rotational speeds. Apolishing pad 203 is attached to the polishing platen 201. The polishingpad 203 is used for polishing a specimen 204 that is held against thepolishing pad 203 with a specimen holder 205. The specimen holder 205can be rotated about an axis, which is parallel to the rotation axis ofthe polishing platen 201, with an actuator 206. The actuator 206 canrotate the specimen holder 205 at different rotational speeds. As thepolishing platen 201 and the specimen holder 205 are rotated, thepolishing pad 203 polishes the specimen 204.

The polishing device comprises three moisture sensors 207 for detectingthe moisture in the polishing pad 203. The moisture sensors 207 arearranged at regular intervals from each other and attached with rods 208to an arm 209 that is located above the polishing pad 203. The arm 209extends from the centre to the periphery of the polishing pad 203.

Each of the moisture sensors 207 comprises electrical contact elements210 and 211, which are arranged in contact with the surface of thepolishing pad 203. The electrical contact elements 210 and 211 providean electrical path through which electrical properties of the polishingpad 203 can be measured. The electrical contact elements 210 and 211 areelectrically connected to a measuring instrument 212 that can measurethe resistance between the electrical contact elements 210 and 211. Theresistance between the electrical contact elements 210 and 211 isindicative of the moisture in a measurement position on the polishingpad 203. The measuring instrument 212 is electrically connected to thepolishing platen 201, thus allowing to measure the capacitance betweenone of the electrical contact elements 210, 211 and the polishing platen201, i.e. between the opposite sides of the polishing pad 203. Thecapacitance between the electrical contact element 210 or 211 and thepolishing platen 201 is indicative of the moisture in a measurementposition on the polishing pad 203.

The polishing device comprises three spray nozzles 213 for dispensing apolishing suspension on the polishing pad 203. The spray nozzles 213 areattached at regular intervals to an arm 214 that is located above thepolishing pad 203. The arm 214 extends from the centre to the peripheryof the polishing pad 203.

The polishing suspension is stored in a container 215 from which it isconveyed through flexible hoses 216 to the spray nozzles 213 to bedispensed on the polishing pad 203. The polishing device comprises acontrol unit 217 for controlling the dispensing of the polishingsuspension. The control unit 217 controls the spray nozzles 213 todispense suitable amounts of the polishing suspension to desiredpositions on the polishing pad 203. The amount of the polishingsuspension to be dispensed to each position is determined based on thedetermined moisture.

Only advantageous exemplary embodiments of the invention are describedin the figures. It is clear to a person skilled in the art that theinvention is not restricted only to the examples presented above, butthe invention may vary within the limits of the claims presentedhereafter. Some possible embodiments of the invention are described inthe dependent claims, and they are not to be considered to restrict thescope of protection of the invention as such.

1. A device for polishing a specimen, comprising: a polishing platenrotatable about an axis, a polishing pad attached to the polishingplaten, and a specimen holder for holding the specimen against thepolishing pad, wherein the device further comprises: means for measuringa physical quantity in a plurality of positions on the polishing pad,the physical quantity being indicative of moisture or friction, andmeans for dispensing a polishing suspension, based on values of themeasured physical quantity, to a plurality of positions on the polishingpad.
 2. The device according to claim 1, wherein the physical quantityis the resistance on the polishing pad, and the means for dispensing apolishing suspension is configured to dispense the polishing suspensionto a position on the polishing pad if in said position the value of theresistance is larger than a first threshold value.
 3. The deviceaccording to claim 1, wherein the physical quantity is capacitancebetween opposite sides of the polishing pad, and the means fordispensing a polishing suspension is configured to dispense thepolishing suspension to a position on the polishing pad if in saidposition the value of the capacitance is smaller than a second thresholdvalue.
 4. The device according to claim 1, wherein the physical quantityis friction on the surface of the polishing pad, and the means fordispensing a polishing suspension is configured to dispense thepolishing suspension to a position on the polishing pad if in saidposition the value of the friction is larger than a third thresholdvalue.
 5. The device according to claim 1, wherein the means formeasuring a physical quantity comprises a sensor arranged in contactwith the surface of the polishing pad, and means for moving the sensorbetween the plurality of positions on the polishing pad.
 6. The deviceaccording to claim 5, wherein the means for moving the sensor comprisesan arm arranged above the polishing pad and an actuator for moving thesensor along the arm.
 7. The device according to claim 1, wherein themeans for measuring a physical quantity comprises an array of sensorsarranged in contact with the surface of the polishing pad.
 8. The deviceaccording to claim 5, wherein the sensor comprises at least twoelectrical contact elements arranged in contact with the surface of thepolishing pad, and the means for measuring a physical quantity comprisesmeans for measuring the resistance between two of the at least twoelectrical contact elements.
 9. The device according claim 8, whereinthe means for measuring a physical quantity comprises means formeasuring capacitance between one of the at least two electrical contactelements and the polishing platen.
 10. The device according to claim 5,wherein the means for measuring a physical quantity comprises acompression element for pressing the sensor against the surface of thepolishing pad.
 11. The device according to claim 5, wherein the meansfor measuring a physical quantity comprises means for measuring thelateral force acting on the sensor.
 12. The device according to claim 1,wherein the means for dispensing a polishing suspension comprises aspray nozzle arranged above the polishing pad and means for moving thespray nozzle between the plurality of positions on the polishing pad.13. The device according to claim 12, wherein the means for moving thespray nozzle comprises an arm arranged above the polishing pad and anactuator for moving the spray nozzle along the arm.
 14. The deviceaccording to claim 1, wherein the means for dispensing a polishingsuspension comprises an array of spray nozzles arranged above thepolishing pad.
 15. A method for polishing a specimen, comprising:holding the specimen with a specimen holder against a polishing pad thatis attached to a polishing platen; rotating the polishing platen aboutan axis; measuring a physical quantity indicative of moisture orfriction in a plurality of positions on the polishing pad; anddispensing a polishing suspension, based on values of the measuredphysical quantity, to the plurality of positions on the polishing pad.